1. Field of the Invention
The present invention relates to an optical circuit and a method for fabricating the same.
2. Description of the Related Art
Request for a large capacity of data transmission have been increased. A parallel transmission has been discussed for carrying out the data transmissions between computer terminals, between switches and between large computers, in a real time and in parallel. Also, the provision of an advanced information service to a home has been discussed. The spread of optical communication is desired for the sake of such large capacity of data transmission.
For the optical communication is used an optical module composed of optical elements such as an optical fiber, a semiconductor laser (LD), a light emission diode (LED), a photo-diode (PD), an optical switch, an optical isolator and an optical waveguide. The application field of the optical element used in such an optical module is expanded since its property and its function are made higher. The cost-down of not only an individual optical element but also the optical module is required to provide the advanced information service to the home. In order to attain the cost-down, the optical module is desirably fabricated in not a coaxial type module structure in which optical elements are arrayed in a form of block, but a flat type optical module structure in which a plurality of optical elements are arrayed on a same substrate.
FIG. 1 shows an optical circuit of a bi-directional communication module in a first conventional example. A semiconductor laser LD 102, a photo-diode PD 103, an optical waveguide 104, a wavelength filter 105 and an optical fiber 106 are mounted on a Si substrate 101. Light having the wavelength of 1.3 μm is emitted from the semiconductor laser LD 102 as a light source to the optical waveguide 104, travels through the wavelength filter 104 and then transmitted to a reception side through the optical fiber 106 as a transmission path. Signal light having the wavelength of 1.5 μm is sent through the optical fiber 106, and is inputted to the optical waveguide 104. The optical path of the signal light is changed to an adjacent waveguide by the wavelength filter 105, and then inputted to the photo-diode PD 103. Thus, the reception of the signal light is carried out. In this way, a small transmission/reception optical module can be attained by use of the flat optical circuit. Grooves are formed on the Si substrate 101 by use of the well-known semiconductor processing technique and used to position the optical waveguide 104, the wavelength filter 105 and the optical fiber 106.
As the optical elements are contained optical elements such as a surface light emission element, a surface light reception element, an end surface light emission element and an end surface light reception element. In the surface light emission element and the surface light reception element, an optical axis is oriented to a direction vertical to a substrate surface. In end surface light emission element and the end surface light reception element, an optical axis is oriented to a direction horizontal to a substrate surface. When two kinds of the optical elements in which the directions of the optical axes are orthogonal to each other are mixed and mounted on the same substrate, it is necessary to change or convert the optical path by 90 degrees. Masataka Itoh et al., (46-th, Electronic Component & Technology Conference, p.1) (a second conventional example) propose an optical path change technique. In this technique, as shown in FIG. 2, an optical path of output light from the optical fiber 106 is changed into the direction of the photo-diode PD 103 by reflecting the output light on a slant surface 109 formed by an anisotropic etching of the silicon substrate 101. In this method, however, the substrate material is limited to the silicon. Thus, the method cannot be applied to other substrates.
A prism for an optical path change is known from Japanese Laid Open Patent Application (JP-A-Heisei, 7-159658) (a third conventional example), as shown in FIG. 3. An optical path of a light beam 107 emitted from the optical waveguide 104 is changed by 90 degrees by a prism 108 or a reflection surface 109 of a reflection mirror. In this case, if the prism having the size of 1 mm or less is used, a fabrication cost of the prism is expensive to further increase a fabrication cost of the optical module.
FIG. 4 shows an installation example of an optical element when the optical path is not changed (a fourth conventional example). In order to mount the light receiving surface of a photo-diode PD 103 in a direction orthogonal to the surface of a substrate 101, it is necessary to newly carry out position adjustment three-dimensionally. Also, it is necessary to add another substrate 110 for supporting the photo-diode PD 103 and a part for fixing the substrate at the adjusted position. Thus, the fabrication cost is further increased.
Light emitted from a light emission element such as a light emission diode and a laser diode spreads to have a certain radiation angle. Even if a waveguide or an optical fiber is closely disposed near the emission portion of the light emission element, there may be a case that the whole emission light cannot be received. Such a case results in an optical loss. For the reduction in the optical loss, a small lens having an excellent light collection performance needs to be used. However, it is difficult to fabricate such a small lens.
In conjunction with the above-mentioned description, Japanese Laid Open Patent Application (JP-A-Showa, 59-7916) discloses a system for coupling a laser diode and a single mode of optical fiber. According to this reference, a curved surface is formed on one end surface or both end surfaces of a self-collection type lens. The lens is disposed between the laser diode and the single mode of optical fiber.
Also, Japanese Laid Open Patent Application (JP-A-Heisei, 7-159658) discloses a coupling structure between an optical waveguide and an optical element. In this reference, the optical waveguide is formed by laminating the dielectrics different from each other. The optical waveguide is formed on a dielectric substrate. In front of an end of the optical waveguide on the side on which the optical element is disposed, a groove is formed on the dielectric substrate to have a bottom surface parallel to a surface of the optical waveguide. A prism is disposed in a groove in which an optical axis of the optical waveguide is coincident with an optical axis of the disposed optical element. The optical element is mounted on the dielectric substrate so that it strides over the prism and the optical waveguide. Metallic coat is formed on one surface of the prism and the bottom surface of the groove. Moreover, a solder sheet is disposed on the surface of the metal coat on the bottom surface of the groove, or the solder layer is formed thereon. The prism is disposed in such a manner that the surface of the prism on which the metal coat is performed faces to the surface of the sheet or the top surface of a solder layer. The dielectric substrate and the prism are heated to thereby melt the solder. Thus, the prism is coupled to the dielectric substrate.
A light connection integrated circuit is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 9-8273). In this reference, each of first and second reflective optical elements has at least three planes orthogonal to each other and a plane parallel to one of the three planes, and further has a reflection plane that is orthogonal to the two planes parallel to each other and disposed for the other plane at a preset angle. Each of the first and second reflective optical elements is formed of transparent material. A flat substrate has a flat surface on which the first and second reflective optical elements are disposed. A first integrated circuit having a light emission device for outputting an optical signal and a second integrated circuit having a light reception device for receiving the optical signal are disposed on a flat surface opposite to the flat substrate of the integrated circuit substrate. The first reflective optical element converts an orientation of the optical signal outputted by the light emission device of the first integrated circuit into an orientation parallel to the flat surface of the flat substrate. The second reflective optical element changes the orientation of the optical signal parallel to the flat surface of the flat substrate so that it is inputted to the light reception device of the second integrated circuit.
Also, an optical path changing method is disclosed in Japanese Patent No. 2,687,859. In this reference, a micro lens is disposed on a mount substrate or a sub-substrate different from the mount substrate. An outer side of the micro lens is used as a light reflection surface. Then, the optical path is converted by about 90 degrees from a horizontal direction to a vertical direction, or reverse. The outer surface of the micro lens may be a spherical surface, or metal coating may be performed on the surface of the micro lens.